The present invention relates generally to high-speed data communications, and specifically to methods and systems for rejection of radio frequency (RF) noise in Digital Subscriber Line (DSL) modems.
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire. This is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. DSL thus opens the most critical bottleneck in local-loop access to high-speed networks, such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) networks, without requiring major investments in new infrastructure.
A range of DSL standards have been defined, known generically as xe2x80x9cxDSL,xe2x80x9d wherein the various standards have different data rates and other associated features but share common principles of operation. These standards include High-Speed DSL (HDSL), at relative low frequencies ( less than 500 kHz); Asymmetric DSL (ADSL), with an intermediate frequency range (30-1100 kHz); and Very High Speed DSL (VDSL), in a high frequency range (0.3-20 MHz). VDSL modems support the highest possible data rates on existing twisted-pair wire, with downstream bit rates of up to 50 Mb/sec. It is expected that in the near future, VDSL systems will come to dominate subscriber equipment on the telephone network.
One problem of VDSL implementation is that the frequency band of VDSL (up to 20 MHz) overlaps with several RF bands used for amateur and commercial radio transmissions. Twisted-pair wire is particularly prone to picking up radio frequency interference (RFI) from external sources. To avoid interference from existing RF systems, VDSL modems must be designed with reduced sensitivity to RFI. This problem becomes more severe the higher the transmission frequency and can cause significant degradation of VDSL signals.
Bodies concerned with defining VDSL standards have published documents that relate to issues of RFI and suggest possible solutions. For example, document T1.E1.4/95-142, promulgated by the T1E1.4 Technical Subcommittee Working Group of the American National Standards Institute (ANSI) and incorporated herein by reference, presents balance measurements of typical VDSL drop wires. The document describes typical power levels of RFI that are encountered. It concludes that longitudinal conversion loss (LCLxe2x80x94representing RFI crosstalk from the common to the differential mode) of 30-35 dB should be taken into account in designing receivers to deal with interference from radio transmissions. The RFI amplitude in the differential signal may thus be as high as 0 dBm (300 mV). Interference between VDSL signals and radio transmissions is also described in other technical papers, such as T1E1.4 Technical Subcommittee Working Group document T1.E1.4/96-025, which is likewise incorporated herein by reference.
The most problematic kind of interference stems from single side band (SSB) modulated transmissions, as are used in amateur radio. Amateur band SSB transmissions are modulated by human speech, which can be modeled as a series of syllables interrupted by short quiet periods. Therefore, the interfering signals are generally stationary only over very short terms. Any noise cancellation that is used must typically adapt to changes in the RFI level within about a millisecond.
T1E1.4 Technical Subcommittee Working Group document T1.E1.4/96-084, which is incorporated herein by reference, describes stationarity properties of RFI sources. The document suggests a method of mitigating short-term stationary RFI in VDSL signals by implementing an adaptive noise canceling module. The suggested method uses a least mean square (LMS) algorithm to rapidly update the noise canceler during periods of silence of the VDSL signals. The document notes that adaptive updating of the module will typically suffer from a problem known as high eigenvalue spread, which slows convergence and exacerbates dynamic range needs of the signal processing. A number of possible solutions are suggested, which may alleviate but do not eliminate the problem.
Methods of noise cancellation for use in DSL systems are also described in PCT Patent Application PCT/US97/06381, published as WO 97/40587, whose disclosure is incorporated herein by reference. This application shows a receiver system for high-speed data communications, such as ADSL or VDSL, having a RF noise canceller. The noise canceller adaptively estimates the radio frequency noise coming into the receiver through twisted pair input lines. The estimate is used to generate a noise cancellation signal, which is subtracted from the signals coming into the receiver. The noise estimate is based on a common-mode reference noise signal, which is sampled at a transformer that couples the input lines to the receiver.
It is an object of some aspects of the present invention to provide improved methods and apparatus for mitigating RFI in high-speed data receivers, and particular in VDSL modems and systems.
It is a further object of some aspects of the present invention to provide a RFI canceller that rapidly tracks and adapts to changes in the characteristics of RFI signals, particularly signals that are short-term stationary, such as SSB signals.
It is yet a further object of some aspects of the present invention to provide a RFI canceller module for use in a data receiver, wherein the module operates substantially autonomously of other data receiver functions, and shuts off when RFI is absent.
In preferred embodiments of the present invention, a high-speed data receiver receives signals over an input line. The receiver comprises an adaptive, narrow-band RFI canceller module, which samples noise on the input line, typically common mode noise arising due to ambient RFI. The module comprises an adaptive bandpass filter (BPF), which rapidly locks onto a narrow frequency range of the strongest RF component in the sampled noise and filters out data and wideband noise that are outside this range. The module processes the resultant narrowband interference signal in order to derive a noise cancellation output, which is subtracted from the signals received over the input line.
Preferably, the module uses a normalized least-mean-square (LMS) method to determine filter coefficients by which the narrowband interference signal is multiplied in order to generate the noise cancellation output. This method converges rapidly to an optimal coefficient value and is therefore particularly useful when the RFI is only short-term stationary. The RFI canceller module is thus optimized to reject the type of common mode interference that arises due to radio transmissions picked up by the line, substantially without introducing any signal degradation outside the interference band.
In some preferred embodiments of the present invention, the adaptive BPF comprises a notch filter element and a bandpass filter element, coupled in parallel to receive the sampled input noise. The notch filter element is used to determine the frequency of the strongest RF component in the sampled noise, by adjusting the notch filter parameters to minimize the notch filter output power. The bandpass filter element uses this determined frequency, and optionally other filter parameters, as well, to lock onto the frequency range of the strongest RF component.
In some preferred embodiments of the present invention, the module comprises a shutoff mechanism, which monitors the amplitude of the RFI and shuts off the noise cancellation output when the amplitude drops below a given minimum level. Preferably, the mechanism monitors aspects of the narrowband signal passed by the BPF, most preferably by checking the values of the above-mentioned filter coefficients. When the absolute values of the coefficients rise above a certain threshold, it is an indication that the RFI has dropped below the minimum level, and the output is shut off. In this manner, it is ensured that the module will not inadvertently cancel the signal or amplify wideband common mode noise occurring on the line.
In some preferred embodiments of the present invention, the data receiver comprises a DSL modem, typically a VDSL modem, which receives signals over a twisted pair line. The RFI canceller module samples a common mode input from the line to the modem. Most preferably, the module samples signals in a common mode choke at the input to the modem, as described in a U.S. patent application entitled xe2x80x9cMethod and Apparatus for RF Common-Mode Noise Rejection in a DSL Receiver,xe2x80x9d filed on even date, which is assigned to the assignee of the present patent application, and whose disclosure is incorporated herein by reference. Alternatively (although generally less desirably), the module may sample the common mode at substantially any other suitable point in the modem, such as from a transformer that couples the line to the modem, as described in the above-mentioned PCT Patent Application PCT/US97/06381. Typically, the RFI detected and canceled by the module arises due to amateur or commercial radio transmissions, although narrow-band noise due to other sources will also be tracked and canceled.
In preferred embodiments of the present invention, the RFI canceller module digitizes the sampled noise, and applies digital filtering and signal processing techniques thereto. In some of these preferred embodiments, the module applies the filtering and processing to the digitized RF noise samples without downconversion, and the noise cancellation output is subtracted from the digitized RF signals received by the receiver. In these embodiments, an accelerated, normalized LMS method is preferably used to determine the filter coefficients, in order to overcome possible problems of eigenvalue spread. In other preferred embodiments, however, the noise and signals are down-converted, preferably to baseband, and the filtering, processing and subtraction take place at the baseband or other reduced frequency. Although these preferred embodiments are based on digital processing, it will be understood that the principles of the present invention may similarly be implemented using analog processing techniques, or a combination of analog and digital processing elements, as are known in the art.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a high-speed receiver, adapted to receive signals over an input line in the presence of narrowband radio frequency interference (RFI), including:
a RFI canceller module, which is coupled to receive samples of the interference from the input line, and which includes:
an adaptive bandpass filter, which is adapted to track a dominant frequency of the interference, and to generate a filtered output in a narrow frequency range centered on the dominant frequency; and
a corrector, which is coupled to receive the filtered output from the adaptive bandpass filter and to generate, responsive thereto, an estimate of the interference in the signals received by the receiver; and
a subtractor, which is adapted to subtract the estimate of the interference from the signals received by the receiver so as to generate a corrected output signal.
Preferably, the signals include differential signals received on the line, and the RFI canceler module is coupled to sample common-mode noise on the input line, and to generate the estimate of the interference responsive thereto. Further preferably, the line includes a twisted-pair cable, and the receiver is included in a Digital Subscriber Line (DSL) modem, wherein most preferably, the signals include Very High Speed DSL (VDSL) signals.
Typically, the interference arises from radio transmissions in a vicinity of the line, including single side band transmissions, wherein the interference arising from the radio transmissions is no more than short-term stationary.
In a preferred embodiment, the receiver includes a downconverter, which is configured to downconvert the samples of the interference before processing thereof by the module. Preferably, the corrector is adapted to generate estimated phase and quadrature components of the interference, and wherein the subtractor includes a pair of subtractors, which are configured to subtracted the phase and quadrature components of the interference, respectively, from phase and quadrature components of the signal.
In another preferred embodiment, the module is adapted to process the samples of the interference substantially without downconversion thereof, and the receiver includes a downconverter, which is configured to downconvert the corrected output signal generated by the subtractor.
Preferably, the receiver includes:
a main analog/digital converter, coupled to generate digitized samples of the signal; and
an auxiliary analog/digital converter, coupled to generate digitized samples of the interference, which are received by the RFI canceller module, wherein the filter, corrector and subtractor operate on the digitized samples.
Preferably, the corrector is adapted to determine a variable filter coefficient using a normalized least-mean-square method and to apply the coefficient to the filtered output from the adaptive bandpass filter so as to generate the estimate of the interference. Further preferably, the RFI canceller module includes a shutoff mechanism, which is adapted, responsive to a value of the filter coefficient, to inhibit subtraction of the estimate of the interference from the signals. Most preferably, the shutoff mechanism is configured to inhibit the subtraction when the value of the filter coefficient rises above a predetermined threshold.
Alternatively or additionally, the RFI canceller module includes a shutoff mechanism, which is adapted to inhibit subtraction of the estimate of the interference from the signals when an amplitude of the narrowband interference on the line drops below a given minimum level.
There is also provided, in accordance with a preferred embodiment of the present invention, a high-speed receiver, adapted to receive signals over an input line in the presence of radio frequency interference (RFI), including:
a corrector, which is coupled to receive samples of the interference from the input line and to generate, responsive to the interference, an estimate of the interference in the signals received by the receiver;
a subtractor, which is adapted to subtract the estimate of the interference from the signals received by the receiver so as to generate a corrected output signal; and
a shutoff mechanism, which is adapted to inhibit subtraction of the estimate of the interference from the signals when an amplitude of the interference on the line drops below a given minimum level.
Preferably, the shutoff mechanism is adapted to inhibit and to re-initiate the subtraction substantially autonomously.
Typically, the interference at least intermittently includes a narrowband interference component, and the shutoff mechanism is adapted to inhibit the subtraction when the narrowband interference component drops below the given minimum level.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for processing signals received over an input line in the presence of narrowband radio frequency interference (RFI), including:
tracking a dominant frequency of the interference;
filtering the interference so as to generate a filtered output in a narrow frequency range centered on the dominant frequency;
generating, responsive to the filtered output, an estimate of the interference in the signals received over the line; and
subtracting the estimate of the interference from the received signals so as to generate a corrected output signal.
There is moreover provided, in accordance with a preferred embodiment of the present invention, a method for processing signals received over an input line in the presence of radio frequency interference (RFI), including:
generating, responsive to samples of the interference received from the input line, an estimate of the interference in the signals received over the line;
subtracting the estimate of the interference from the received signals so as to generate a corrected output signal; and
automatically inhibiting subtraction of the estimate of the interference from the signals when an amplitude of the interference on the line drops below a given minimum level.
There is additionally provided, in accordance with a preferred embodiment of the present invention, an adaptive bandpass filter, including:
a notch filter element, coupled to receive an input signal having a strong narrowband component, the notch filter element having one or more variable filter parameters that are adjusted so as to minimize an output of the notch filter element responsive to the input signal; and
a bandpass filter element, coupled to filter the input signal responsive to the adjusted parameters of the notch filter element so as to preferentially pass the narrowband component of the signal.
Preferably, the one or more variable filter parameters include a variable center frequency of the notch filter element, which is adjusted so as to minimize the output of the notch filter, thereby determining a dominant frequency of the narrowband component. Further preferably, the bandpass filter element is adapted to pass the input signal within a narrow frequency range centered on the dominant frequency determined by the notch filter.
There is moreover provided, in accordance with a preferred embodiment of the present invention, a method of adaptive bandpass filtering, including:
receiving an input signal having a strong narrowband component;
filtering the input signal using a notch filter having one or more variable notch filter parameters, so as to generate a notch filter output;
adjusting the one or more variable notch filter parameters responsive to the input signal so as to minimize the notch filter output; and
filtering the input signal using a bandpass filter having at least one variable bandpass filter parameter, which is adjusted responsive to the one or more adjusted variable notch filter parameters.